The preparation of diesters of oxalic acid (oxalates) is of particular interest to the chemical industry owing to the varied uses of these compounds. Not only may these diesters serve to act as the starting materials for such important compounds as oxalic acid, oxamide or ethylene glycol but they may also find extensive use as intermediates for dyes, pharmaceuticals, and the like.
Prior to the instant invention, there have been proposed numerous processes for the preparation of diesters of oxalic acid employing various catalysts, co-catalysts, reaction accelerators, and the like, for use in a liquid phase reaction to make oxalates. However, these conventional processes suffer from significant by-product formation as would be expected from the conventional liquid phase processes.
A particularly interesting attempt to prepare diesters of oxalic acid in the liquid phase is disclosed in U.S. Pat. No. 4,138,587. This patent employs nitric acid or a nitrogen oxide as an accelerator in the presence of a solid platinum group metal or salt, thereof, molecular oxygen, an alcohol and carbon monoxide to produce the diester of oxalic acid. Unfortunately, the process is a liquid phase process and suffers in several significant practical aspects, such as catalyst losses by virtue of dissolution, large by-product formation, low efficiencies to product, to name a few.
U.S. Pat. No. 4,229,591 discloses a vapor phase process. The process disclosed in the patent involves contacting an ester of nitrous acid with carbon monoxide in the gaseous phase under normal pressure in the presence of a solid catalyst containing metallic palladium or a salt thereof at a temperature of 50.degree. to 200.degree. C., the ester of nitrous acid being nitrous acid ester of an alcohol having 1 to 8 carbon atoms.
Although this above-described process is advantageous as compared to liquid phase processes the process fails to distinguish the role played by the carrier for the catalysts employed in such a heterogeneous vapor phase process. This is better shown by reference to the example of the patent. Examples 1 to 24 depict various palladium catalysts but in each case the carrier for the palladium catalyst was either carbon or SiO.sub.2. The specification refers also to alumina, diatomaceous earth, pumice, zeolite, and molecular sieves. Obviously, the broad general listing of carriers fails to signify any advantage of one carrier over another. Carbon and silica (SiO.sub.2) carriers are acidic carriers with high surface areas (much greater than 10 m.sup.2 /g). For example, the patent mentions "alumina" as a carrier, and this encompasses a wide variety of materials ranging from high surface area acidic-alumina (gamma-alumina), fibrous alumina, to alpha-alumina.